In a conventionally employed method for producing a 3,4-dihydrocoumarin derivative, a 3-(2-cyclohexanoyl)-propionic acid ester derivative is heated in the presence of a hydrogenation-dehydrogenation catalyst such as palladium to perform ring formation and dehydrogenation. In the above reaction, a coumarin derivative is formed as a by-product. This by-product coumarin derivative is separated from the reaction mixture and then partially hydrogenated with hydrogen in the presence of a fresh hydrogenation-dehydrogenation catalyst so as to convert the coumarin derivative into a dihydrocoumarin derivative, and the resulting dihydrocoumarin derivative is recovered (as described, e.g., in U.S. Pat. No. 3,442,910 and J. Am. Chem. Soc., vol. 62, p.p. 283-287 (1940)).
The catalyst for use in the partial hydrogenation reaction may be one obtained by regenerating the catalyst that has been used in the ring formation and dehydrogenation reaction. The regeneration of the catalyst is accomplished by (1) subjecting the catalyst to hydrogen reduction at a high temperature (Shokubai Sekkei (Catalyst Design), edited by Shokubai Gakkai, published by Kodansha, Japan) or (2) subjecting the catalyst to cracking at a high temperature (as described, e.g., in U.S. Pat. Nos. 2,506,307 and 2,532,615).
The above-described conventional methods are defective in that both the by-product coumarin derivative and the catalyst should be separated from the reaction mixture, and that it is necessary to conduct the regeneration of the catalyst at a high temperature (about 480.degree. to 600.degree. C.) in either of the above methods (1) and (2). Thus, the separation of the reaction product and the regeneration of the catalyst necessitate troublesome procedures and are uneconomical. In addition, the regeneration method (1) above is dangerous because hydrogen is used at a high temperature.